Hydrogels are attractive for biomedical applications.
Further, hydrogels exhibiting the specific ability of increasing their viscosity with temperature, also called “thermosensitive/thermoresponsive/pseudothermosetting/thermogelling hydrogels”, were proved to have a facilitated application combined with an increased residence time at the site of application, and therefore may be advantageously used for drug delivery or tissue augmentation.
As known from O. Felt et al. in The Encyclopedia of Controlled Drug Delivery, 1999, said thermosensitive hydrogels may be based advantageously on polymers of natural origin, for example on chitosan which is a commercially available inexpensive polymer derived from chitin, the second most abundant polysaccharide after cellulose.
Chitosan is known as a chitin derivative obtained by partial to substantial alkaline N-deacetylation of chitin also named poly(N-acetyl-D-glucosamine), which is a naturally occurring biopolymer.
Chitosan contains free amine (—NH2) groups and may be characterized as to the proportion of N-acetyl-D-glucosamine units and D-glucosamine units, and such is expressed as the degree of deacetylation (DD) of the fully acetylated polymer chitin.
Parameters of chitosan influencing important properties such as solubility and viscosity are the degree of deacetylation (DD) which may be understood as representing the percentage of deacetylated monomers, and the molecular weight (Mw).
Chitosan is known to be biodegradable, biocompatible, bioadhesive, bacteriostatic, and further to promote wound-healing, drug absorption, and tissue reconstruction.
Due to its above mentioned intrinsic properties, chitosan is known to have numerous cosmetic and pharmaceutical activities, and has been also widely explored for various applications through gels.
Therefore, considering the advantageous properties of chitosan, there is a continuous need to improve the properties of known thermosensitive chitosan hydrogels which are still considered as very promising for a wider range of biomedical applications.
WO-A-99/07416 (BIOSYNTHEC) discloses a pH-dependent temperature-controlled chitosan hydrogel which has thermosensitive properties at neutral pH such that it has low viscosity at low temperature but gels at body temperature.
This thermosensitive chitosan hydrogel is prepared by neutralizing a commercial chitosan having a deacetylation degree of about 80% with mono-phosphate dibasic salts of polyols or sugars exemplified in particular by β-glycerophosphate (β-GP).
However, presence of β-GP in the hydrogel leads to the following disadvantages.
β-GP is a negatively charged entity that can react with a positively charged bioactive component, leading to its precipitation or to the disturbance of its liberation from the hydrogel.
Therefore, presence of β-GP renders chitosan/β-GP hydrogels inappropriate for use with numerous drugs.
Further, the properties of this hydrogel, such as gelation time and viscosity, depend on the concentration of β-GP and are therefore limited by the solubility of β-GP.
In particular, a high concentration of β-GP is required to have a low gelation time avoiding the rapid elimination of the hydrogel after its administration.
However, a high concentration of β-GP also decreases the mechanical properties of the hydrogel.
Therefore, the gelation time has to be balanced with the consistency of the hydrogel, and it is not possible to obtain gels that have both a low gelation time and a high viscosity, which would be a desirable combination of characteristics.
Also, a too high concentration of β-GP may induce the precipitation of the hydrogel at its administration site.
Further, said thermosensitive chitosan/β-GP hydrogels were found to be turbid, thus rendering their use inappropriate for particular applications such as ocular or topical administrations.
In addition, phosphate-containing materials may be inappropriate in terms of biocompatibility (G. Molinaro et al., Biomaterials, 23:2717-2722 (2002)).
In order to overcome the disadvantages of chitosan/β-GP hydrogels, it was proposed in WO-A-2005/097871 (UNIVERSITE DE GENEVE) a thermosetting neutralized chitosan composition forming a phosphate-free transparent hydrogel at a temperature higher than 5° C., said composition comprising an homogeneously reacetylated chitosan having a molecular weight of not smaller than 200 kDa, and a deacetylation degree of 30-60%, neutralized with an hydroxylated base.
WO-A-2005/097871 also disclose that said composition may further comprise 1,3-propanediol to modulate the viscoelastic properties of the hydrogel.
However, 1,3-propanediol is neither mentioned as “generally recognized as safe” (GRAS), nor recognized as an additive mentioned in US pharmacopoeia, European pharmacopoeia or Japanese pharmacopoeia so that its use is limited in biomedical applications.
Further, thermosetting neutralized chitosan compositions containing 1,3-propanediol lose their thermoresponsive properties upon lyophilization.
In view of the continuous heed to provide improved thermosensitive chitosan hydrogels for biomedical applications, the present inventors have continued their researches to overcome the disadvantages of the known thermosensitive hydrogels.
An object of the present invention is to provide an aqueous thermosetting neutralized chitosan composition forming a phosphate-free transparent hydrogel having improved properties and being acceptable for biomedical applications.
Another object of the present invention is to provide an aqueous thermosetting neutralized chitosan composition which may be stored easily and which preserves its thermogelling properties after storage.
Still another object of the present invention is to provide an aqueous thermosetting neutralized chitosan composition having a facilitated application, for example by injection using needles or minimally invasive techniques.
These objects are achieved by the present invention.